For tourists, Florence is very crowded in the summer, and there will be a line to enter the Duomo, but don’t be discouraged. It is a magnificent structure of outstanding craftsmanship that could never be recreated in modern times. Inside the dome Vasari painted an enormous fresco. You can hike to the top of the dome via narrow stairway in between the internal and external walls if you wish; it’s a workout!
The story behind the Duomo is fascinating. I recommend Ross King’s “Brunelleschi’s Duomo,” a wonderful book that reads like a novel, not a history textbook. It tells the story of the competing architects who designed and built the enormous and ornate Duomo beginning in 1418. You will learn that no one had yet figured out how to build this large a vaulted dome over the foundation that was complete up to the base of the dome, and how they did it.
It is difficult to photograph the entire church from either the front or back, because the Baptistry, another architectural gem with bronze relief doors by Ghiberti, stands directly in front, and other buildings rise along the sides and rear. You can best see the famous dome while walking away from the church down a narrow cobbled street. Inside and out, from the sidewalk or top of the dome, the Duomo and its story are extraordinary.